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Science: Martian Slopes Sprout Dark Streaks in Warm Seasons

Researchers have identified new surface features that appear on Mars’ slopes during the warm season and fade in the cold season. These clusters of dark lines could potentially be formed by flowing, salty water, though the observations reported in this study do not prove that.

In their report in the 12 August issue of Science, Alfred McEwen of the University of Arizona’s Lunar and Planetary Laboratory and colleagues analyzed images taken by the High Resolution Imaging Science Experiment (HiRise) on the Mars Reconnaissance Orbiter. The images cover a variety of latitudes and span a period of approximately three martian years.

The researchers spotted these features, which they call “recurring slope lineae,” reaching down steep slopes such as the rims of impact craters, usually on the equator-facing slopes.

University of Arizona student Lujendra Ojha, at the time a junior majoring in geophysics, first detected the features during an independent study project.

“I was baffled when I first saw those features in the images after I had run them through my algorithm,” said Ojha, who is a co-author on the Science publication. “We soon realized they were different from slope streaks that had been observed before. These are highly seasonal, and we observed some of them had grown by more than 200 meters in a matter of just two Earth months.”

The researchers identified seven sites in the mid-latitudes of the southern hemisphere where these features occur in multiple warm seasons and fade in cold seasons. They also identified similar features at a variety of sites at other latitudes, though it’s not yet clear whether these change over time in the same way.

In their paper, the researchers consider a range of possible scenarios for what’s forming these features, and they propose that briny water—which could stay liquid at the temperatures these slopes reach during their warm season—could be responsible. The exact mechanism for how this would work and the water’s source are not yet clear, however.

“The best explanation we have for these observations so far is flow of briny water, although this study does not prove that,” said McEwen.


Read the abstract, “Seasonal Flows on Warm Martian Slopes,” by Alfred McEwen and colleagues.

Listen to a Science Podcast interview with Alfred McEwen.